The present invention is in the field of gear pumps. More particularly the present invention relates to a gear pump wherein a pair of meshed straight-cut spur gears are received within a housing having an inlet port and an outlet port leading to and from a cavity wherein the meshed gears are received. One of the meshed gears is driven by an output shaft extending externally of the housing, while the other gear is journaled in the housing as an idler and rotates because of its meshing engagement with the externally driven gear. As the meshed gears rotate in opposite directions successive trapped volumes of fluid are carried by each gear from the inlet passage to the outlet passage. The mesh of the gears prevents fluid from being conveyed in the opposite direction.
Such meshed gear pumps are old and very well known. For example conventional gear pumps are known wherein a housing portion of the pump is adjusted transversely relative to the gears in order to control radial clearance between the housing portion and the tips of the gear teeth. Such gear pumps are set forth in U.S. Pat. No. 246,724 issued 6 Sept. 1881 to A. Clark; and in U.S. Pat. No. 3,433,168, issued 13 Jan. 1967 to O. H. Baker; as well as in U.S. Pat. No. 4,645,439, issued 24 Feb. 1987 to D. R. Way. Also known are conventional gear pumps wherein a portion only of the housing is adjusted transversely of the gear pair in order to control the radial gap between the tips of the gear teeth and the housing portion. Such a conventional gear pump is set forth in U.S. Pat. No. 2,855,854, issued 19 Feb. 1954 to L. L. Aspelin, which teaches that the laterally moving housing portion may be arranged as a pressure balanced component of the gear pump. Still further U.S. Pat. No. 3,560,121, issued 28 Feb. 1969 to G. L. Noell, teaches that the laterally moving portion of the housing may be arranged as a pair of tilting pads each individually engaging one of the meshed gears of the pump and being pressure balanced as a pair to move laterally toward the meshed gears and thereby control the radial clearance between the tilting pad members and the tips of the gears.
In addition to the above discussed teachings which have to do with control of leakage within a gear pump by control of the radial clearances between the gears and a portion or portions of the gear pump housing, U.S. Pat. No. 2,986,097, issued 30 May 1961 to R. S. Chrzanowski et.al. teaches that a gear pump may be provided with a housing secured axially by a pair of tie bolts or tie rod members extending parallel to the shafts of the meshed gears and exceedingly close to the mesh of these gears. These tie bolts preload the housing to resist axial separating forces generated by fluid pressures within the pump. In this way it is hoped to avoid the axial bowing apart of the housing portions of the pump resulting from fluid pressures within the pump. Such separation of the housing portions provides leakage past the meshed gears axially thereof.
An alternative method of providing control of axial clearances within a meshed gear pump is set forth in U.S. Pat. No. 3,748,063, issued 24 July 1973 to R. C. Putnam, wherein an axially movable pressure-balanced plate member is disposed against one axial face of each of the meshed gears. The pressure-balanced plate member biases the gears axially into sliding and sealing engagement with the opposite face of the pump housing, and itself moves into sliding and sealing engagement with the meshed gears in response to controlled pressure forces effective thereon.
However, each of the above referenced gear pumps is believed to suffer from a deficiency only apparent when such gear pumps are subjected to operation at exceedingly low temperatures. Such operation of gear pumps at exceedingly low temperatures occurs in the aerospace art as well as in other arts. By way of example only, in the aerospace art it is desirable for equipment which is carried aboard an aircraft at high altitude and which, therefore, may be exposed to exceedingly low temperatures as low as -65.degree. F., to be able to start and operate successfully at these low temperatures. Unfortunately, conventional gear pumps when they are exposed to this type of dormant cold soak utilization experienced an exceedingly high drag torque when they are first started. This drag torque continues for a considerable period of time until the pumps approach their normal operating temperature, and in extreme cases this drag torque, which may be a virtual lock-up of the pump, may entirely prevent the starting or operation of the associated equipment upon which the gear pump is used.
The applicants have discovered that this undesirable characteristic of conventional gear pumps is largely attributable to the conflicting requirements in the design of such a pump. On the one hand sufficiently close radial and axial clearances must be maintained within the pump so that leakage flows are kept to an acceptable minimum. On the other hand, acceptably large clearances must exist after a cold-soak so that the rotating and relatively sliding parts within the pump are not brought into forceful engagement with one another by the thermal contraction experienced during such cold soaking.
In view of the above it is apparent that a gear pump which is largely unaffected by cold soak as well as operation at high temperatures while still maintaining desirably close radial and axial clearances is highly desirable.